1. Field of the Invention
The present invention relates to imaging systems, and particularly to hyperspectral imaging systems suitable for airborne deployment.
2. Discussion of the Known Art
Because of security concerns, there is an increasing demand for systems capable of remotely detecting potentially dangerous objects such as explosives or toxins from a safe distance. Airborne hyperspectral imaging systems can be used to determine the composition of these and other objects as well as their physical properties. Such systems combine two-dimensional image sensing technology with a hyperspectral dispersing technique to provide a three-dimensional remote sensing capability. Emitted or reflected light or electromagnetic radiation is collected from the object through optics of the system, and is separated into spectral components or wavelength bands. Because the spectral components are typically unique to the material or element of which the object is composed, various properties of the object may be determined and the object identified by analyzing the separated spectral components. Hyperspectral data sets usually contain many contiguous bands of high spectral resolution over a region of the electromagnetic spectrum. See generally, X. Prieto-Blanco, et al., “Analytical design of an Offner imaging spectrometer”, Optics Express, vol. 14, no. 20 (October 2006), at pages 9156-68 and incorporated by reference; and F. Reininger, Caltech/NASA JPL, “Optics for Compact, High-Performance Imaging Spectrometers”, at <http://m3.jpl.nasa.gov/docs/Offner_spectrometer.pdf>.
U.S. Pat. No. 5,880,834 (Mar. 9, 1999) discloses an imaging system having fore-optics in the form of a three-mirror anastigmatic telescope (TMA), and an imaging Offner spectrometer. The fore-optics forms an intermediate image at a slit before the spectrometer. An off-axis primary spectrometer mirror delivers radiation onto a secondary spectrometer mirror/diffraction grating, and a tertiary spectrometer mirror reflects light from the grating to form multi-spectral images on a detector surface. Further, U.S. Pat. No. 6,100,974 (Aug. 8, 2000) discloses an imaging system including fore-optics also in the form of a TMA, and a dispersive Offner spectrometer consisting of three mirrors decentered with respect to one another. All relevant portions of the mentioned '834 and '974 U.S. patents are incorporated by reference.
Another imaging spectrometer is described in an article by C. Simi, et al., “Compact Airborne Spectral Sensor (COMPASS)”, Proc. of SPIE, vol. 4381 (2001), at pages 129-36. The COMPASS system is comprised of fore-optics in a form of a three-mirror anastigmat, and an Offner spectrometer with a single or dual blaze grating etched on a curved surface. The system is disclosed as being compact, off-axis, and able to deliver 256 spectral channels while having an F-number of 2.5.
Yet another imaging spectrometer is described in an article by J. Yiquan, et al., “Compact hyperspectral imaging system with a convex grating”, Proc. of SPIE, vol. 6834, 68340Y (2007), at pages 1-9. The system includes a three mirror anastigmat telescope and an Offner imaging spectrometer with a convex diffraction grating. As disclosed, the system is relatively large, has an F-number of 2.5, and delivers modest performance. All relevant portions of the Simi, et al. and the Yiquan, et al. articles are incorporated by reference.
The known hyperspectral imaging systems have certain significant drawbacks when deployed in airborne and/or military applications, however. For example, optical components of the fore-optics and/or components of the imaging spectrometer must define certain decentrations or tilts relative to one another to obtain correct optical alignment. The tilts and decentrations are highly sensitive to minor variations from predetermined values. Thus, the assembly, alignment, and testing of the systems is difficult, time consuming, and costly. Some of the systems also have a relatively high F-number, tending to reduce system sensitivity and, therefore, mission capability. Moreover, their optics may have a relatively small field of view (FOV), thereby reducing target coverage and/or requiring faster scanning.
Accordingly, there is a need for a high performance hyperspectral imaging system that is compact, provides good performance with a fast (low) F-number, and is easier to fabricate, align and test than existing systems.